Softball and baseball leagues have experienced a dramatic increase in the number of bats being altered by players to enhance hitting performance. The most common method for altering a bat to increase performance is a practice known as “rolling,” in which the bat barrel is placed between two cylinders (“rollers”) that are oriented perpendicularly to the longitudinal axis of the barrel. The rollers are compressed into the bat barrel, which deflects the bat cross section. (A schematic diagram of a rolling setup is shown in FIG. 2.) While the barrel is in the compressed mode, the bat is moved along its longitudinal axis through the compression rollers to compress the barrel along most of its length. This rolling is typically repeated at least 10 times and is generally performed approximately every 45° around the barrel's circumference.
To obtain increased performance, players generally repeat the rolling process at a deflection significant enough to break down the shear strength between plies in the barrel, which severely alters the barrel kinetics. The mechanism by which this is achieved is generally referred to as accelerated break-in (“ABI”).
Methods to induce ABI generally target the weak interlaminar region of the composite structure, which leads to interlaminar fracture or delamination. Delamination is a mode of failure that causes composite layers within a structure to separate, resulting in significantly reduced mechanical toughness of the composite structure. The strength at which a composite structure fails by delamination is commonly referred to as its interlaminar shear strength. Delamination typically occurs at or near the neutral axis of the barrel laminate and serves to lower the barrel compression of the bat, which increases barrel flex and “trampoline effect” (i.e., barrel performance). While following this procedure shortens the bat life, players commonly elect a temporary increase in performance over durability.
For many softball bats, approximately 0.20 inches or more of ABI rolling deflection may be required before the barrel initially fails and performance increases. The actual amount of deflection required depends upon the overall durability of the barrel design: the more durable the barrel design, the more deflection the barrel can withstand without performance increases. Less durable laminate designs, conversely, may only withstand approximately 0.10 inches of deflection, for example, before barrel performance increases.
To help prevent the use of impermissibly altered bats, the Amateur Softball Association (“ASA”) has implemented a new test method that requires all softball bats to comply with performance limits even after the bats are rolled an unlimited number of times. The ASA requires a bat to remain below a chosen performance limit (currently 98 mph when tested per ASTM F2219) or break during the test. Sufficient breakage of the bat needs to be notable by the players or umpires on the field.
The NCAA has recently adopted a similar ABI protocol for composite baseball bats. The protocol uses ASTM F2219 to measure the performance level of the bat calculated as bat-ball coefficient of restitution (“BBCOR”). This protocol requires rolling of a bat to test for performance increases that might occur when a bat is overstressed or damaged. The BBCOR and barrel compression are tested when the bat is new and undamaged. If the bat tests below the established performance limit, the bat is then subjected to rolling. If the barrel compression changes by at least 15%, the bat BBCOR is retested. If the barrel compression does not change by 10%, the bat is rolled again with the deflection increased by 0.0125″. This cycle is repeated until a bat exceeds the performance limit or passes the protocol. To pass the protocol, a bat must show a decrease of a least 0.014 in ball exit speed ratio (“BESR”) or 0.018 in BBCOR, or the bat must break to a point where testing the bat can no longer provide a measurable rebound speed.
The dramatic increase in players altering bats has forced associations to test composite bats all the way through failure to assure they do not exceed performance limits at any time. With this turn of events, the focus of bat design must adapt.